This invention relates to an improved chain extender blend of critical proportion such that its liquid state is maintained at room temperature for direct mixing with a polyol reactant at ambient temperature and the chain extending profile of the blend is suitable for large mold reaction injection molding. The invention is particularly directed to the preparation of urethane/urea polymer by the RIM or RRIM process the basic principles of which were developed in Germany by Bayer AG. Typically, this process has been used to produce elastomeric urethanes in molded parts employed in production of fascia for automobiles, recreational vehicles, manufacture of shoe soles and chemically resistant coating compositions. The RIM process involves the production of moldings having a good performance and superior processing efficiency by a one step process. Generally, it is carried out by introducing a reactive polymerizable mixture, based on compounds having several reactive hydrogen atoms, such as polyethers and polyesters having terminal hydroxy groups, and isocyanates, such as diphenylmethane diisocyanate, tolylene-2,4- and 2,6-diisocyanates or polymeric diphenylmethane diisocyanate or mixtures thereof, into a mold. The reaction mixture essentially contains the polyol and isocyanate reactants, a chain extender and catalyst.
The RIM method wherein the reaction mixture is rapidly polymerized in the mold is opposed to the substantially slower castable, process which involves a first isocyanate prepolymerization step, relatively long pot life of the prepolymer to afford time for mixing and reaction with polyol before molding and curing and demolding times of 20-40 minutes. Accordingly, chain extenders used in the castable process such as for example the polyhalogenated toluene diamines, methylene bis(2-chloro-aniline) must be slow acting to accommodate the requirements of the process.
As explained hereinafter the rapid mold polymerization which characterizes the recently developed RIM process has many advantages over the well known time consuming "castable" process. With the advent of (RIM) techniques, perfected in the mid 1970's, it became desirable to have fast acting systems. Therefore, it is not surprising that there are many different aromatic diamines known in the art ranging typically from the slow methylene bis(o-chloroaniline), to the intermediate polymethylene phenyldiamine, to the very fast methylene bis(aniline) and unhindered toluene diamine.
When polyurethane or polyurethane-polyurea forming ingredients are deposited or otherwise injected into a mold cavity, it is essential that a careful balance of polymer gel time versus the molded part size be maintained. A fast enough gel time is necessary so that good polymer formation together with quick demold time is possible. Contingently, the gel cannot be so fast that polymer gelation is occurring on the mold walls or floor while ungelled reactants are still entering the mold. Accordingly, the polyurethane or polyurethane-polyurea polymer must have suitable reaction profiles which can be varied or adjusted by chain extenders to meet the particular needs of the RIM process.
The RIM reaction mixture of polyol, isocyanate and chain extender is characterized by very fast reaction profiles which can be too fast to fill large mold sizes before gelation begins to initiate. This difficulty is encountered with the toluene diamines devoid of electron-withdrawing groups, namely the unsubstituted or alkyl substituted toluene diamines. Attempts to continue injection of such polymer forming ingredients into a mold after gelation commences results in poor polymer flow causing rapid and excessive tool pressure build-up and the potential for the rupture of the tool. Although an increase in the speed of mixing and delivery of the reaction ingredients into the mold can overcome some of these difficulties, there is a practical limit above which laminar flow will be upset. This will result in voids and unequal distribution of polymer in the mold.
It is now discovered that extended elastomeric polyurethane-polyurea polymers, which are characterized by good hardness, tear, improved modulus properties and higher demold strengths, compared to prior art extended urethane/urea polymers, can be prepared by using certain aromatic diamine-diol extender blends in which the diamine is the minor component. With the incorporation of these blends, the polymer-forming reactants have reaction profiles fast enough to be suitable for rapid molding and demolding processes as in the RIM or RRIM applications, yet slow enough to enable the filling of large mold sizes before gelation can occur and thereby avoid the prior art problems noted above. The present blend is ideally suited to the large mold RIM or RRIM process due to its high activity with controlled gel time and facilitated incorporation into the reaction mixture at ambient conditions.
It is an object of the present invention to provide a commercially attractive process for the preparation of polyurethane/polyurea elastomer molds achieved by a RIM or RRIM process.
Another object of this invention is to provide a more efficient and economical chain extender blend for use in preparing molds, and effectively large molds, of polyurethane/polyurea elastomer by the RIM or RRIM process.
Another object is to provide a fast acting chain extender blend which, in admixture with polyol/isocyanate reactants, is not subject to gelation before the reaction mixture completely fills the mold.
Still another object is to provide a chain extender blend which remains liquid at room temperature and is more easily incorporated and uniformly distributed in the polyol/isocyanate reactant mixture before polymerization.
Another object is to provide a molded polyurethane/polyurea product having improved modulus, hardness and impact strength throughout the polymer product.
Still another object is to provide a molded polyurethane/polyurea product having more uniformly distributed cross-linked sites and uniform phase mixture.
Whenever the term "essentially" or "comprising essentially" or "consisting essentially" is used in the specification or in the claims, it is intended to mean that the operative steps or materials to which the terms refer are an essential part of that phase of operation, but that unspecified materials or steps are not excluded so long as they do not prevent the advantages of the invention from being realized.